Not applicable
The present invention relates to a doctor blade for a papermaking machine in general and to a doctor blade constructed of plastic in particular.
Faces of rolls in a paper/board machine tend to be coated with impurities derived from the papermaking process. Doctor blades are used in order to remove these materials from roll faces. As the running speed of paper machines has increased, the amount of friction between the doctor blade and the roll face has also increased, resulting in increased temperature at the doctor blade/roll interface and of the doctor blade itself. This is a problem, because the materials conventionally used in doctor blades do not withstand such higher speeds. For example, at a paper machine speed greater than 1400 meters per minute, doctor blades made of conventional materials can start to melt and abrade rapidly, in which case they no longer operate in cleaning of the roll face.
From the prior art, many doctor blades made of different materials are known, including composite structures. In U.S. Pat. No. 4,549,933, a doctor blade for a paper machine is described, which blade consists of a number of alternating layers of fibre and carbon fibre. The fibre layer can consist of cotton, paper, fibreglass, or equivalents thereof.
On the other hand, in published German patent application DE 4137970, a doctor blade comprising fiber-reinforced plastic is suggested. The fibre-reinforced plastic contains from 60 to 90 per cent by weight of polyamide-6 or polyamide-66, and from 10 to 40 per cent by weight of reinforcement fibers. A polyamide, which is a thermoplastic resin, is used in order to increase the thermal conductivity of the blade.
In Finnish Patent FI 101,637, a caring doctor blade is described, which blade comprises a number of fibre layers in a laminate construction, where at least one layer of carbon fibre or at least one layer that contains a substantial proportion of carbon fibre is present. This patent further discloses that the blade contains grinding particles in direct vicinity of the carbon fibers and that the carbon fibers are oriented substantially obliquely in relation to the direction of the longitudinal axis of the blade, preferably in the cross direction of the blade.
Japanese Published Application JP 05-214696, discloses a doctor blade comprising polyethylene of very high molecular weight or fibre-reinforced polyethylene of very high molecular weight, which polyethylene is a thermoplastic resin.
Japanese Published Application JP 05-32118 describes a doctor blade which is made of a thermoplastic fibre composite material which contains from 30 to 80 percent by weight of polyphenylene sulphide (a thermoplastic resin), and from 20 to 70 percent by weight of either glass fibers, aramide fibers, or graphite fibers.
Finally, Japanese Published Application JP 05-13289 discloses a doctor blade which consists of a material that contains fibreglass, where the filament fibres have been immobilized in a resin parent material, such as epoxy resin.
As evidenced by the above prior art, a number of different thermoplastic resin materials have been suggested for use in a doctor blade. In spite of their desirable heat resistance properties, thermoplastic resins have not achieved commercial importance as doctor blade materials because of their high cost and because of their difficult workability. A thermosetting plastic from which high resistance to heat in operation is expected also requires a considerably high melting-processing temperature. In practice, in commercial products, epoxy resins have been used almost exclusively.
However, doctor blades that comprise an epoxy matrix tend to wear, or degrade rapidly, resulting in shorter service life. As machine running speeds increase, this problem has become even worse. As discussed earlier, higher machine operation speed increases the friction heat between the revolving roll and the doctor blade. This heat causes the epoxy in the doctor blade to soften and start to melt. The phenomenon of softening is increased by the wet conditions, for epoxy has a certain degree of tendency to absorb water. The softening and the melting have the effect that the roll face becomes coated with the blade material. This causes changes in the properties of adhesion, separation and surface energy in the roll face, which has a very detrimental effect on the operation of the papermaking machine.
A second serious drawback of epoxy is its poor suitability for pultrusion and for similar methods that would allow continuous manufacture of doctor blades.
Thus, it is an object of the present invention to provide such a material for a doctor blade that can endure high paper machine running speeds and, thus, high operating temperatures at the doctor blade/roll interface.
It is an additional object of the present invention to provide a doctor blade which can withstand high operating temperatures, and also possesses good mechanical strength and rigidity.
It is yet a further object of the present invention to provide a doctor blade that can be manufactured efficiently in a variety of ways, including continuous manufacturing processes, such as pultrusion.
These, and other objects and advantages, are achieved by the doctor blade of the present invention.
The present invention relates to a doctor blade for cleaning a roll face in a papermaking machine, comprising a thermosetting plastic polymer material selected from the group consisting of vinylesterurethanes and polyether amide resins. Other thermosetting plastic polymers can also be used, provided that their glass transition temperature (Tg) is at least 20xc2x0 C. higher than the operating temperature at the blade/roll face interface at any operating speed of which the papermaking machine is capable of being operated. In addition to being able to endure high operating temperatures, the thermosetting plastic polymers of the doctor blades of the present invention also have high impact resistance. Since these materials do not come close to their Tg temperature during operation, blade wear resulting from softening and/or melting is slower. Also, in such a case, the wear takes place in a controlled way without breaking of the tip of the blade. Controlled wear is important in order that the blade should remain sharp through its whole service life. Owing to high impact strength, the blade tip is not broken equally easily if some material adhering to the roll face passes under the blade in a running situation.
Owing to their nature of thermosetting plastic, the thermosetting plastic polymers for use in the doctor blades of the present invention are suitable for being processed by all methods that are used with thermosetting plastic, including pultrusion. Moreover, processing of these materials does not require considerably elevated temperatures, as the processing of thermoplastic resin materials does. In the manufacture of oblong pieces, such as doctor blades, suitability for pultrusion is a highly desirable feature, because it permits continuous manufacture, in which case the overall economy of the manufacture is better and the product is of uniform quality.
In accordance with a preferred embodiment of the invention, the doctor blades are composite structures further comprising reinforcing materials and/or filler materials. The reinforcing materials can be conventional fibre reinforcements, such as glass, carbon or aramide fibers, or structures woven out of said materials or mixtures of said fibre reinforcements. For example, a multi-layer structure can be made using structure fibreglass and carbon fibre reinforcements, where the alignment of said reinforcement fibers vary/alternate in different layers.
Not applicable.